docs: def _find_geophysical_vars(self, ds, refresh=False):
          '''
          Returns a list of geophysical variables.  Modifies
          `self._geophysical_vars`
  
          :param netCDF4.Dataset ds: An open netCDF dataset
          :param bool refresh: if refresh is set to True, the cache is
                               invalidated.
          :rtype: list
          :return: A list containing strings with geophysical variable
                   names.
          '''
          if self._geophysical_vars.get(ds, None) and refresh is False:
              return self._geophysical_vars[ds]
  
          self._geophysical_vars[ds] = cfutil.get_geophysical_variables(ds)
  
          return self._geophysical_vars[ds]